Inhibitory M2 muscarinic receptors on parasympathetic nerves become nonfunctional in parainfluenza virus infected guinea pigs. While preventing inflammation preserves MS receptor function, the exact inflammatory cell involved cell involved is not known . Macrophages are increased in and activated by viral infection and are involved in antiviral defense. They are likely to be involved in virus-induced M2 receptor dysfunction. The overall goal of this project is to determine the role of the alveolar macrophage in virus-induced M2 receptor dysfunction and airway hyperresponsiveness. Specific aims of this project are: Specific aim #1: To determine whether depleting macrophages will prevent virus- induced airway hyperresponsiveness and M2 receptor dysfunction in virus-infected guinea pigs. Animals will be pretreated with liposome- encapsulated clodronate which leads to macrophage apoptosis. Five days later, macrophage-depleted animals will be infected with parainfluenza virus. Four days later, airway responsiveness to histamine and electrical stimulation of the vagi will be measured and the function of the M2 receptor will be determined. Specific aim #2: To determine whether virus-induced loss of M2 receptor function is mediated via production of nitric oxide. Animals will be pretreated with nitric oxide inhibitor prior to and during viral infection. Airway responsiveness and M2 receptor function will be determined four days after viral infection. Specific aim #2: To determine whether virus-induced loss of M2 receptor function is mediated via production of nitric oxide. Animals will be treated with nitric oxide inhibitor prior to and during viral infection. Airway responsiveness and M2 receptor function will be determined four days after viral infection. Specific aim #3: To determine the direct effects of alveolar macrophages on M2 receptor function and gene expression. Macrophages, from bronchoalveolar lavage, will be added to primary cultures of airway parasympathetic neurons. Release of acetylcholine from these neurons, the ability of the M2 receptor and to inhibit acetylcholine release, and the expression of the M2 receptor gene (measured by competitive RT-PCR) will be determined. I will employ antibodies to block candidate macrophage-derived cytokines to determine their role in the loss of M2 receptor function.